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COMPLEX BRASSES                                  231

iron has been separated is concentrated, with the anode immersed, to 60-
70 c.c. and the manganese determined as indicated in the electrolytic analysis
of ferro-rnanganese.

5.  Determination  of the Zinc.—If not already acid,  the solution
containing the zinc and aluminium is acidified with dilute sulphuric acid,
evaporated if necessary to reduce the volume to 200-250 c.c., rendered
alkaline with sodium hydroxide, acidified slightly with formic acid and
electrolysed with rotating electrodes to determine the zinc (see p. 225).

6.  Determination of the Aluminium.—The liquid from which the
zinc has been removed, together with wash water, is treated with ammo-
nium chloride and ammonia until the reaction is alkaline, boiled for some
time and the aluminium hydroxide filtered off, washed, dried, ignited and
weighed :   Al20n x 0-5303 = Al.

7.  Determination of the Phosphorus, Sulphur, Bismuth, Arsenic,
etc.—See Ordinary Brass.

B.   Combined Gravimetric and Volumetric Method

2 grams of the sample are dissolved in 20 c.c. of nitric acid (D 1-2) at
a gentle heat.

1.  Determination of the Tin, Lead  and  Copper.—The liquid is
diluted with 15-20 c.c. of water, and the tin, lead and copper determined
gravimetrically as in ordinary brass (q.v.).

2.  Determination of the Iron, Manganese, Zinc and Aluminium.
—The liquid from which the copper has been separated as sulphide is
evaporated to reduce its volume to about 200 c.c. and to expel the hydrogen
sulphide, the iron being oxidised by addition of a few drops of hydrogen
peroxide and the cooled liquid made up to 300 c.c.

of this solution is reduced with zinc amalgam and titrated with perman-
ganate in the usual way.

portions, each of 50 c.c., are used for the determination of the manganese
by Volhard's method (see Ferro-manganese), one for the preliminary test
and the other for the actual determination.

(c)  DETERMINATION OF THE ZINC.   A further quantity of 50 c.c. is
rendered alkaline with ammonia and then neutralised with formic acid;
for each 100 c.c. of this solution 4 c.c. of 50% formic acid are added and
the zinc precipitated by hydrogen sulphide.    Ihe liquid is left overnight
to complete the precipitation and is then filtered through a filter-paper of
compact texture and washed with a dilute solution of ammonium formate
saturated with hydrogen sulphide.1

When completely washed, the precipitate is dried and introduced into
a Rose crucible, the filter being burned in a platinum spiral and dropped
into the crucible. The precipitate is then mixed by means of a platinum

1 The ammonium formate solution is prepared by neutralising a dilute solution of
formic acid (1-2%) with ammonia and slightly acidifying with formic acid. possible quantity of 10% sulphuric acid, a few